Natural materials like wood, cotton, wool and even concrete have an
equilibrium
water content that changes with ambient relative humidity. This makes
these materials
conductive but not conductive enough to carry dangerous currents.
They are still good insulators
for mains voltage.
The textiles can change moisture content and conductivity quite
quickly
as they have fibre diameters of about 20 microns or so, but even at
20% RH static charges
dissipate in seconds. Plastics, however, tend to be very non-
conductive and charges
can be held for hours. Also, some plastics form electrets which stay
"charged" even under water.
A great insight to the static electricity problem came from articles
I read as a young boy in old (1910)
articles such as "make yourself an electrophorous" in popular science
mags.
A can lid was filled with melted resin. When solid it was rubbed with
wool. A metal
disk with insulated handle was placed on the resin and grounded with
a finger.
The disk was then lifted off the resin and would be found to have a
high charge (and voltage) on it.
(half inch fat spark to ground)
This process of induced charging and potential multiplication is the
danger on work benches.
The main way to overcome it is to have an isopotential environment
which naturally occurs with
natural materials where all charges rapidly drain away. Wood is good,
it does not produce charge
when rubbed and rapidly drains any charge away. And unless soaking
wet it will not electrocute you
if you are leaning on it when you touch and active power lead. (my
theory from experience is that
it is the ground that would kill you when if you were electrocuted.
If you have good soles on your shoes
and the other hand in your pocket an accidental touch to high voltage
is survivable)
just a few thoughts,
Neville Michie
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